Friday, August 6, 2010

Despite the importance of forests in the global carbon (C) cycle, estimates of coarse root mass in forest ecosystems remain uncertain because of tremendous variability in reported root: shoot (R:S) ratios. Recent meta-analyses find negligible ecological influences on variation in forest R:S ratios, even though experiments and ecological theory suggest otherwise. We argue that a major source of variation in R:S ratios is methodology, which obscures detection of ecological drivers of R:S ratios and leads to underestimates of C storage and sequestration. We compiled 174 R:S ratio samples from 58 published studies on forests and evaluated each study with respect to four methodological criteria: 1) recovery of roots from soil; 2) volumetric extent of excavation; 3) accounting for spatial structure in root biomass; and 4) inclusion of the root crown and tap root.

Results/Conclusions

Satisfaction of each criterion increased geometric mean R:S ratio and studies that met all four criteria had a geometric mean R:S ratio of 0.36 versus 0.24 for studies that failed >1 criteria. Based on studies that met criteria, mean R:S ratio was higher under dry versus mesic soil conditions, marginally higher in natural forests than plantations, and did not differ among boreal, temperate, and tropical forests. Using this refined mean R:S ratio of 0.36 to estimate the C inventory of US forests resulted in increases of 81% in forest root biomass (~9000 Tg CO2 equivalents), 13% in total forest biomass, and 12% in annual C sequestration compared to current estimates that assume a R:S ratio of 0.20. Our refined estimates of R:S ratios should aid in developing a more accurate national carbon inventory.